Principles of Genetics and Breeding

7.1.3.2 Crossing over

The physical exchange of parts of non-sister chromatids of homologous chromosomes following synapsis at meiosis is known as crossing-over. This results in the recombination of genes. 1,or 2, or more fragments may be interchanged during crossing-over.

Example for crossing over

If a female F₁ hybrid for grey body and long wings (Gg Ll) is backcrossed (test cross) with a double recessive male having black body and vestigial wings (gg ll), the results obtained in the F₂ generation are different. Two new combinations are seen in the F₂ generation viz. grey body vestigial wings (Gg ll) and black body long wings (gg Ll). These new combinations appear in equal numbers and together constitute 17% of the F₂ generation. The old combinations also appear in equal numbers and constitute 83% of F₂ generation.

The two new combinations appear in the F₂ generation because the F₁ female produces four types of gametes. Out of these, two types are the normal non-crossovers (GL and gl) which are 83%. The other two types of gametes (Gl and gL) which constitute the remaining 17% are produced as a result of crossing over. Recombination thus occurs in about 17% gametes.

Stage of occurrence

Crossing over occurs during meiosis. During zygotene stage pairing of chromosomes takes place, mutual attraction between parts of chromosomes that are homologous. This is two strand stage. Pachytene stage is tetrad stage. Four strands are formed. Crossing –over takes place. Chiasma is formed. Only two chromatids participate in the formation of chiasma. Chromatids may break in the diplotene stage and then rejoin in such a manner that homologous parts are interchanged between them.

The frequency of crossing over of a given pair of genes is not constant. Temperature, nutrition, sex, age, etc influence the crossing over in several animals. Crossing over frequencies are higher in the female sex than in males.

To conclude crossing over is a genetic mechanism that occur in all living organisms. The genes that occur in linear order is broken by recombination. Crossing over is apart of meiotic mechanism during gametogenesis. This brings about variation and leads to evolution through natural selection.

Kinds of cross-over

Depending upon the number of chiasmata formed, crossing over may be of the following kinds.

1. Single crossing-over

In this case, only one chiasma is formed resulting in the formation of single-cross-over gametes. Only one chromatid of each chromosome is involved in single crossing over. Single crossing over is of most frequent occurrence.

2. Double crossing-over

In this case, two chiasmata are formed. Both the chiasmata may be between the same chromatids or between different chromatids. Thus two, or three, or all the four chromatids of the homologous pairs of chromosomes may be involved in the process of double crossing-over. Double cross-over gametes are produced in this case. This is of less frequent occurrence.

3. Multiple crossing-over

In this case, more than two chiasmata are formed. Corresponding to the number of chiasmata formed, it is called triple (3 chiasmata), quadruple (4 chiasmata), and so on. Multiple crossing-over does not occur frequently.

1. It provides a direct evidence of the linear arrangement of genes in the chromosomes.

2. Chromosome maps can be constructed.

3. It gives rise to new combinations of genes, and hence, variations in offspring.

Significance